Air Entrainment

Biting the Dust

The building owner wanted to fix the floor's dusty condition before moving in.

Cracks developed throughout the floor.

{ QUESTION } The owner of a recently constructed building contacted us to help solve a problem with the concrete floor's surface. The floor is a slab on grade, and appears to be structurally sound. It seems crews followed proper joint patterns and curing procedures.

The building is a manufacturing facility containing a steel cold-forming machine and a heat treat furnace. It is unheated and there are no ventilation problems. There is limited forklift activity and all have rubber tires.

The owner wants to resolve the floor surface's dusty condition, which existed before he moved in. The surface has no chipping or flaking, but there are some random cracks. And in some areas when the surface is slightly wet, there are spider web-like patterns of small microcracks.

The owner is considering applying a surface coating on the floor to mitigate the dust. But I'm concerned this is an expensive solution. I believe it's best to solve the root cause of the dusting before applying a coating of any sort. I want to eliminate any chance that the coating will peel off. Can you offer a solution?

{ ANSWER } How to repair the floor depends on the quality of the concrete. In most cases, the dusting your potential client is experiencing results from the abrasion of a thin surface layer of material.

There are two potential causes. This thin layer probably occurred when the concrete contractor who placed the slab finished it too early. The finishing operation brought fines to the top, which hardened to a rather weak concrete of very low abrasion resistance.

Another potential cause for dusting is a chemical reaction. When there's carbon dioxide in the ambient air, it combines with the calcium silicate hydrate and calcium hydroxide in hydrated cement paste to form calcium carbonate.

According to ACI, when this reaction occurs, the concrete undergoes irreversible carbonation shrinkage, which can be the cause of the surface crazing you mentioned in your question. Also, when freshly placed concrete surfaces are exposed to carbon dioxide from improperly vented combustion heaters (used to keep concrete warm during the winter) during the first 24 hours, they are susceptible to dusting.

The floor's condition appears to be sound. The few random cracks are probably plastic shrinkage cracks. Even so, you should inspect the slab to determine if the control joints are properly located throughout the area.

There are several solutions. One of which is to grind off or shotblast off this thin layer. This will expose the solid concrete underneath and could expose a new surface with good wear resistance.